ICPT power systems (also known as contactless power supplies) are known to have significant advantages in applications such as the materials handling, lighting and transportation industries. There are many applications in both high and low power systems in which use of these power supplies is advantageous.
ICPT systems have a primary conductive path supplied with alternating current from a power supply. One or more secondary devices (which may be referred to as pick-ups) are provided adjacent to, but electrically isolated from, the primary path. The pick-ups have a pick-up coil in which a voltage is induced by the magnetic field associated with the primary path, and supply a load such as an electric motor, a light, or a sensor for example. The pick-up coil is usually tuned using a tuning capacitor to increase power transfer to the pick-up.
A problem with existing ICPT systems is control of the power transferred to pick-ups when they are lightly loaded, for example when a motor is supplied by a pick-up and is idle while it awaits a command from a control system. A solution to this control problem is the use of a shorting switch across the pick-up coil to decouple the pick-up when required and thus prevent flow of power from the primary conductive path to the pick-up. This approach is described in the specification of U.S. Pat. No. 5,293,308 assigned to Auckland UniServices Limited. However, although that specification addresses the control problem of lightly loaded pick-ups, the shorting switch causes large conduction losses, especially at light loads because it is nearly always conducting in light load conditions.
Another problem with ICPT systems is variation in the frequency of the current in the primary path. Frequency drift can cause the primary path current to fluctuate which causes problems with control of power transferred to the pick-ups. More importantly, frequency drift can significantly affect the tuning of pick-ups, especially those that use fixed frequency tuning. This reduces the ability of the system to effectively transfer power. Frequency drift can be caused by many factors. The most obvious is load change, but circuit parameter variations can also be significant.
One approach to compensate for frequency variations in the primary conductive path caused by load changes is to provide a plurality of individual capacitors and switch individual capacitors into or out of the primary power supply circuit. This approach has been posed in recently published United States patent application US2003/0210106. This has disadvantages in high Q systems because many capacitors are required. Also, load variations have to be limited to make the system function effectively.
Another approach is to use a more complex power supply, such as a third-generation (G3) supply, for the primary conductive path. This is expensive and such power supplies are not suited to miniaturisation.